Typically, bodies of vehicles, etc. are manufactured by pressing a metal thin plate into a pre-designed die. However, it is impossible in practice to manufacture them using a lightweight die made of a thick metal plate, etc. having various curved surfaces in the aspects of cost, time and so on.
General curved surface forming methods for a metal plate can be roughly classified into a cold forming method and a hot forming method. The hot forming method comprises bending a steel plate using the characteristic of the steel plate such that it shrinks locally when locally heated and rapidly cooled. For example, a method of working a ship steel plate generally employs two methods: hot working and cold working. Cold working causes plastic deformation of material by applying mechanical force to the material at room temperature using a press or a roller, whereas hot working bends a steel plate using the characteristic of the steel plate in which local shrinkage occurs when it is locally heated and rapidly cooled. Since the steel plate shows good machinability in a heated state, the hot working method of heating and bending the steel plate is frequently used. At present, the hot working mainly uses a line heating method using a gas torch etc. However, since this line heating method is highly dependent on the proficiency and experience of a worker, recently it has been very difficult to maintain uniform quality due to the aging and attrition of skilled workers. Furthermore, this line heating method is impossible to use in conjunction with a computer system, and thus has a limitation in the improvement of working efficiency.
The cold working method employs a bending roller and a hydraulic press. The bending roller includes three or four rollers disposed in a vertical direction. A steel plate is inserted into the gap between an upper roller and a lower roller and is pressed using a hydraulic jack coupled with the upper roller, and the rollers are rolled to bend and push out the steel plate. This process is repeated several times until a desired curved surface is formed. This cold working method is mainly suitable for bending plates having a two-dimensional curved surface among the external plates of a ship hull. In the case of a three-dimensional curved surface, a steel plate is roughly bent by the aforementioned process, and then is again worked such that a desired curved surface is obtained by the line heating method. Further, the hydraulic press is connected with a hydraulic apparatus, and is used to press the curved plate to form a desired curved surface. In the case in which mass production is required, the steel plate is hydraulically pressed by a stationary die module which is designed and manufactured in advance. In contrast, in the case in which small quantity batch production is required, meaning the case in which the number of curved plates to be worked is small, the stationary die module is manufactured at an enormous cost, and thus is impractical to apply in reality.
Recently, the demand for three-dimensional curved surface forming of various industrial metal plates such as ship steel plates for offshore plants for deep seabed mining and other various industries has increased sharply. At present, these curved plates are mainly formed by the hot working method, and thus encounter the difficulties described above. When the cold working method is applied, the number of workable workpieces is limited. In the case in which small quantity batch production is required, it is impossible in practice to design and manufacture the stationary die module. As a known useful method capable of overcoming this problem, a so-called multi-point forming method has been proposed. A forming apparatus applying this technology has been manufactured and is industrially available at present. The technical gist of the known multi-point forming method incorporated into the present invention is disclosed in “Review on Basic Forming Principle” (Study on Multi-point Sheet Forming Method, Vol. 1, M C Lee, et al), pages 519 to 522 of Japan Plastic Working Spring Lecture Meeting (May 24˜26, 1992, Yokohama Japan), “Failure in Multi-point Forming and its Control” (Study on Multi-point Sheet Forming Method, Vol. 3), and pages 425 to 428 of the 43rd Japan Plastic Working Spring Lecture Meeting (Oct. 1˜3, 1992, Tokyo Japan).
The multi-point forming method is a kind of cold working technology, to which a hydraulic press is basically applied, is a method of continuously arranging a series of steel punches, called forming punches, setting a die having a similar shape to a target curved surface, and carrying out forming through hydraulic pressing. Since this method can alter the shape of the die as needed, even if the number of workpieces to be worked is small as described above, various curved surfaces can be worked using a single forming apparatus. Moreover, the working environment for the forming is remarkably good compared to that of the existing hot working, and the existing hydraulic press can be used without change.
However, when a metal plate is plastic-worked by the hydraulic press, spring-back inevitably occurs. “Spring-back” refers to a phenomenon in which part of the metal plate elastically recovers upon deformation. In the complicated three-dimensional curved surface forming of the metal plate, spring-back is extremely complicated. The extent of spring-back varies according to the position within the curved surface. In spite of the many technical merits described above, when the multi-point forming method is used to form the complicated curved surface of the metal plate without checking the effects of spring-back in detail in advance, extensive work experience is required for precise formation. In order to obtain a target curved surface, a skilled worker must repeat the hydraulic pressing several times. As a result, there is a possibility of causing local damage to the worked workpiece. During forming, intermediate processes for checking whether or not the target curved surface is obtained are required. In this manner, a sophisticated, complicated process must be conducted by a highly skilled worker. Consequently, it takes a long time to manufacture the workpiece, and the quality of the formed workpiece varies according to the proficiency of the worker.
Thus, the development of technology capable of completely overcoming the problems with the multi-point forming method is acutely required. Moreover, although the demand for small quantity batch production for forming curved surfaces on metal plates has recently increased, the curved surface formation is still dependent on the experience of skilled workers. Thus, a limitation on the efficiency of production stems from the shortage of workers.